(1) Field of the Invention
This invention relates to a radiographic apparatus for acquiring fluoroscopic images of an object under examination by emitting radiation thereto, and more particularly to a radiographic apparatus having a radiation grid for removing scattered radiation generated in the object.
(2) Description of the Related Art
Medical institutions have radiographic apparatus installed therein for picking up fluoroscopic images of patients. As shown in FIG. 15, such a radiographic apparatus 51 includes a top board 52 for supporting a patient M, a radiation source 53 for emitting radiation, and a radiation detector 54 for detecting the radiation.
The radiation detector 54 has a radiation grid 55 placed on a radiation incidence plane (detecting plane) thereof for removing scattered radiation produced from the patient M. The radiation grid 55 has elongated strips of absorbing foil arranged as in a blind. When the scattered radiation strikes on the absorbing foil strips, most of the radiation is absorbed by the absorbing foil instead of reaching the radiation detector 54. Thus, the scattered radiation is absorbed and removed by the radiation grid 55.
The radiation grid 55 allows clear fluoroscopic images to be acquired free of the scattered radiation, but gives rise to the following problem. That is, shadows of the absorbing foil strips will fall on the radiation detector 54. Then, a striped pattern with dark pixel values will be reflected on a fluoroscopic image, which worsens visibility of the fluoroscopic image.
Some conventional radiographic apparatus are constructed to remove this striped pattern by image processing. That is, a conventional radiographic apparatus carries out a frequency analysis of a fluoroscopic image to remove the striped pattern. The striped pattern appearing on the fluoroscopic image has a plurality of dark lines arranged at constant intervals. When a frequency analysis is conducted on the fluoroscopic image, components the striped pattern will collect at a certain frequency. A fluoroscopic image free of the striped pattern will be acquired when a frequency inverse transform is carried out after a process for removing the frequency components. Such construction is described in detail in Japanese Unexamined Patent Publications No. 2000-83951 and No. 2002-257939.
Such a striped pattern removing method can be used also for a synchronous radiation grid. The synchronous radiation grid will be described hereinafter. The radiation detector 54 has detecting elements arranged in a matrix form on the detecting plane thereof for detecting radiation. The synchronous radiation grid is a radiation grid manufactured by arranging absorbing foil strips so that the pitch of the shadows of the absorbing foil strips falling on the radiation detector 54 may be an integral multiple of the pitch of the detecting elements (see Japanese Unexamined Patent Publications No. 2000-83951 and No. 2002-257939). When the synchronous radiation grid is used with a radiation source and a radiation detector set to a standard position, there will occur no moire due to interference between the pitch of the detecting elements and the pitch of the shadows of the absorbing foil strips. Thus, a fluoroscopic image can be acquired with less disturbance on the image due to the shadows of the absorbing foil strips. Even if the synchronous radiation grid is used, a striped pattern, though less conspicuous, still appears on the fluoroscopic image. This will be removed by frequency analysis.
However, the conventional radiographic apparatus has the following drawback.
According to the conventional radiographic apparatus, it is difficult to remove completely the striped pattern appearing on a fluoroscopic image. As shown in FIG. 15, where the radiation source 53 and radiation detector 54 are supported by a C-arm 57, a rotation of the C-arm 57 can incline the radiation source 53 and radiation detector 54 as maintained in the same relative position. When picking up images using such a radiographic apparatus, the operation is carried out while inclining the C-arm 57. The C-arm 57 can bend at this time to shift the relative position of the radiation source 53 and radiation detector 54 slightly. This will shift the position of the shadows of the absorbing foil strips appearing on the detecting plane of the radiation detector 54.
The conventional type radiation grid has the absorbing foil strips arranged in order with spacers such as of graphite interposed between the absorbing foil strips. These spacers absorb radiation to some extent. The synchronous radiation grid has nothing between adjacent absorbing foil strips, but it is hollow there to lessen the absorption of radiation. On the other hand, the absorbing foil strips can be distorted to deviate from accurate linearity because of the hollow structure. That is, winding shadows of the absorbing foil strips will be projected to the detecting plane of the radiation detector 54. Even so, the shadows of the absorbing foil strips are located in the middles in a transverse direction of the detecting elements. The shadow of each absorbing foil strip does not cover the detecting element pairs adjoining in the transverse direction, but appears in the fluoroscopic image as a vertical line having a width of one pixel.
If an image is picked up with the C-arm inclined from this state, the shadow of each absorbing foil strip will move in the transverse direction. Since the shadow of each absorbing foil strip is winding, the shadow will partly cover or will not cover adjacent detecting elements from the transverse direction. Then, the shadows of the absorbing foil strips appear as a complicated striped pattern on the fluoroscopic image. It is difficult to remove such a striped pattern by frequency analysis. The striped pattern becomes complicated, and difficult to remove uniformly by frequency analysis.
This invention has been made having regard to the state of the art noted above, and its object is to provide a radiographic apparatus which can reliably remove a striped pattern superimposed on a fluoroscopic image due to shadows of absorbing foil strips of a radiation grid falling on a radiation detector.